FIG. 5 shows a constitution of a conventional surface acoustic wave filter. In FIG. 5, reference numeral 101 is an input terminal, 102 is an output terminal, 103 is a piezoelectric substrate, 105 to 108 are resonators provided on a signal line 104a on the piezoelectric substrate 103, and this surface acoustic wave filter is composed of a comb-shaped electrode pattern (not shown) and reflector patterns (not shown) provided at its both ends. Further, reference numeral 109 is a package, 110a to 100f are lands formed in the package 109, and 111a to 111f are wires for connecting the resonators 105 to 108 in the signal line 104a and a ground pattern 104b.
Herein, by composing the rcsonators 105 to 108 on the piezoelectric substrate 103 by means of the comb-shaped electrode pattern and reflector patterns provided at its both ends, an elastic vibration is induced to the surface. By operating as the resonators by making use of this elastic vibration, the frequency filter, that is, the surface acoustic wave filter is obtained.
In this constitution, however, when a large input signal is applied to the input terminal 101, a sudden temperature rise takes place in the resonators 105 and 106, in particular, closer to the input terminal 101, and it may cause migration in the metal composing the comb-shaped elcctrode pattern (usually aluminum or aluminum alloy), thereby leading to precipitation, oxidation or deterioration, and the desired filter characteristic might not be obtained.
That is, the conventional surface acoustic wave filter has been known as a filter generally capable of obtaining a steep filter characteristic, but it was designed to handle a small signal and limited in the range of use, and hence it was difficult to handle a large signal in the surface acoustic wave filter